Jockey Kelly Castaneda averted serious injury in a spill at Laurel Race Course yesterday, five days after his brother broke a wrist on the same track.

Castaneda, 33, went down an instant after the gates opened for the ninth race. Classic Rival stumbled in her first stride, flipping Castaneda onto the dirt. He landed on his head and right shoulder in front of Classic Rival, who might have clipped him as she proceeded.

Castaneda was taken to Greater Laurel-Beltsville Hospital and released last night. Track physicians had suspected a possible concussion, but a hospital spokeswoman said Castaneda was treated primarily for muscle strains. It was not known when he would resume riding.

Castaneda began the day with one victory from 19 mounts this meet, having ended a lengthy winless streak on Tuesday.

His older brother, Marco Castaneda, broke his right wrist in a two-horse spill on Saturday and probably will be sidelined six to eight weeks. McWhorter Wins First

With a little daring, Bobby McWhorter won his first race in Maryland yesterday.

The apprentice from Thistledown began advancing Karnak On The Nile from last place in the fourth race when favored Poolside Chatter broke down. While Mark Johnston battled to keep Poolside Chatter upright, McWhorter took a chance and pushed Karnak On The Nile between the limping horse and the rail.

"I could see Mark was trying to get out of everyone's way," McWhorter said. "There was no sense trying to go around him and getting caught wide. I figured it was a good chance to take."

They made it through cleanly, after which McWhorter directed Karnak On The Nile toward the outside. The 3-year-old continued his charge between horses and won by three-quarters of a length over Prince Of Spades, paying $14.20. . . .

Jockey Mario Pino had a miserable day, getting two horses disqualified -- A Joyful Guy from first place in the third race, Music of the Sea from second in the 10th. Pino could receive a seven-day suspension for each offense. . . .

The Maryland Racing Commission has fined veterinarian Byron Young a total of $1,200 for violating state racing rules. Young was fined $1,000 for administering medications not approved by the U.S. Food and Drug Administration and $200 for failing to submit medication reports. . . .

The double-triple carryover remains at $32,677 going into today's program after the stewards invoked a rarely used rule concerning a late scratch yesterday.

Horses were going into the gate for the fifth race when S.J.'s Maverick became anxious in his stall and fell. State veterinarian David Zipf determined him unfit to race.

The stewards concluded there was not enough time for bettors to exchange any live double-triple tickets that included S.J.'s Maverick, and therefore distributed the amount wagered on yesterday's double-triple pool among those holding live tickets. Each of the 105 tickets returned $78.60. . . .

Laurel canceled plans to simulcast Saturday's Dwyer Stakes after only four horses were entered in the $150,000 race at Belmont Park.